Formation of Ohmic contacts in III-nitride light emitting devices

ABSTRACT

P-type layers of a GaN based light-emitting device are optimized for formation of Ohmic contact with metal. In a first embodiment, a p-type GaN transition layer with a resistivity greater than or equal to about 7 Ωcm is formed between a p-type conductivity layer and a metal contact. In a second embodiment, the p-type transition layer is any III-V semiconductor. In a third embodiment, the p-type transition layer is a superlattice. In a fourth embodiment, a single p-type layer of varying composition and varying concentration of dopant is formed.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional of application Ser. No.09/755,935, filed Jan. 5, 2001, which is a continuation-in-part ofapplication Ser. No. 09/092,065, filed Jun. 5, 1998. application Ser.Nos. 09/755,935 and 09/092,065 are incorporated herein by reference.

BACKGROUND

[0002] 1. Field of Invention

[0003] The present invention is related to the manufacture of III-Vlight emitting and laser diodes, particularly towards improving thecharacteristics of the electrical contact to the p-type portion of thediode.

[0004] 2. Description of Related Art

[0005] Gallium nitride (GaN) compounds have wavelength emissions in theentire visible spectrum as well as part of the UV. FIG. 1 illustrates atypical GaN-based light emitting diode (LED). Currently, most GaN-basedLEDs are epitaxially grown on a sapphire or silicon carbide (SiC)substrate. A double hetero-structure that includes a nucleation layer,n-type layer, active region, p-type AlGaN layer, and a p-type layer ofGaN is formed on the substrate. In general, the ability to fabricateohmic contacts to the p-type layer is essential for the realization ofreliable light emitting diodes and laser diodes. Ohmic contacts top-type GaN are difficult to achieve because the attainable holeconcentration is limited for Mg-doped III-nitride based semiconductors.In addition, many light-emitting diodes and vertical cavitysurface-emitting laser diodes use thin, transparent metal contacts. Thechoice of metals is limited and metal layers need to be thin, e.g. <15nm, to reduce light absorption. Because there is poor lateral currentspreading in p-type GaN, the metal layers typically cover nearly theentire device area.

[0006] P-type conductivity for GaN is achieved by doping with Mg, whichsubstitutes for gallium in the GaN lattice and acts as an acceptor(Mg_(Ga)). Mg_(Ga) introduces a relatively deep acceptor level into theband gap of GaN. As a consequence, only ˜1% of the incorporated Mgacceptors are ionized at room temperature. To illustrate, a Mgconcentration ([Mg]) of ˜5e19 cm⁻³ is needed to achieve a roomtemperature hole concentration of ˜5e17 cm⁻³. Further, Mg-doped GaNrequires a post-growth activation process to activate the p-typedopants. The post-growth activation process may be, for example, thermalannealing, low-energy electron-beam irradiation, or microwave exposure.For conductivity-optimized Mg-doped GaN layers, [Mg]<5 e19 cm⁻³, theacceptor concentration (N_(A)) is about equal to the atomic Mgconcentration and the resistivity can be around 1 Ωcm or less. Theselayers may be referred to as “p-type conductive layers”. Increasing theMg content beyond approximately 5e19 cm⁻³ does not translate to higheracceptor concentration. Typically, a reduction of N_(A) is observed whenthe [Mg] exceeds a certain maximum concentration and the layer becomesresistive.

SUMMARY

[0007] P-type layers of a III-nitride-based light-emitting device areoptimized for formation of an Ohmic contact with metals. In someembodiments, a p-type transition layer is formed between a p-typeconductivity layer and the metal contact. The p-type transition layermay be a GaN layer with a resisitivity greater than 7 ohm-centimeters, aIII-nitride layer, a III-nitride layer with added As or P, or asuperlattice with alternating highly doped or elemental dopant sublayersand lightly doped or undoped sublayers.

[0008] In some embodiments, the p-type layer is continuous with varyinglevels of dopant. The concentration of dopant in the region of thep-type layer adjacent to the p-contact is greater than the concentrationof dopant in the region of the p-type layer adjacent to the activeregion. The p-type layer may also have a varying composition, forexample of Al or In or both.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 illustrates a prior art light-emitting diode.

[0010]FIG. 2 illustrates a light-emitting diode according to a firstembodiment of the present invention.

[0011]FIG. 3 illustrates N_(A) plotted as a function of [Mg].

[0012]FIGS. 4A and 4B demonstrate the I-V characteristics for aNi/Au—Mg-doped GaN contact in “back-to-back” configuration for themetals deposited on a p-type conductive layer (A) and on a p-typetransition (B) layer.

[0013]FIG. 5A demonstrates the relationship between p-contact barrierheight and resistivity for Mg-doped GaN layers.

[0014]FIG. 5B demonstrates the effect of contact annealing on theNi/Au—Mg-doped GaN contact barrier for p-type conductive and for p-typetransition layers where the p-type conductivity was activated by twodifferent RTA (5 min) activation processes (600° C. and 850° C.).

[0015]FIG. 6 demonstrates the relationship between bandgap energy andlattice parameter for the AlInGaN material system.

[0016]FIG. 7 illustrates a light-emitting diode according to a thirdembodiment of the present invention.

[0017]FIG. 8 illustrates a light-emitting diode according to a fourthembodiment of the present invention.

[0018]FIG. 9 illustrates the variation of Al and In composition and Mgconcentration across the p-type layer of several examples of oneembodiment of the light-emitting diode illustrated in FIG. 8.

DETAILED DESCRIPTION

[0019]FIG. 2 schematically illustrates a GaN light-emitting diode 10according to a first embodiment of the present invention. A nucleationlayer 12 is grown over a substrate 14, for example A1 ₂O₃, SiC, or GaN.An n-type layer 16 of GaN that is doped with Si is fabricated over thenucleation layer 12. An active region 18 of InGaN is fabricated over then-type layer 16. A p-type layer 20 of AlGaN:Mg is fabricated over theactive region 18, followed by a p-type layer 22 of Mg-doped GaN that hasbeen optimized for conductivity (p-type conductivity layer), followed bya p-type transition layer 24 deposited over the p-type layer 22. Metalcontacts 26A and 26B are applied to the n-type layer 16 and the p-typetransition layer 24, respectively. The metal contacts may be transparentor opaque.

[0020] P-type transition layer 24 is optimized to form a good Ohmiccontact with the metal layer. In the first embodiment, the material ofp-type transition layer 24 is a GaN-based layer that contains a higheratomic Mg but a lower acceptor/hole concentration when compared to thep-type conductivity layer 22. In FIG. 3, the dependence of N_(A) isillustrated as a function of [Mg]. Curve 30 illustrates Mgconcentrations and resulting acceptor concentrations for a specific setof growth conditions. Other growth conditions may cause the curve toshift up or down or left or right, but the shape of the curve isexpected to be approximately the same as curve 30 regardless of thegrowth conditions. As illustrated by curve 30, when a GaN-based film ishighly doped with Mg, the acceptor concentration decreases, thus thefilm becomes highly resistive. This behavior is atypical of other III-Vsemiconductors.

[0021] Exemplary Mg and acceptor concentrations for p-type conductivitylayers are shown in region 34. Typically, p-type conductivity layer 22has a [Mg] less than approximately 5e19 cm−3, N_(A)˜[Mg], andresistivities of about 1 Ωcm or less. In contrast, p-type transitionlayer 24 is a highly resistive film having a [Mg]>about 5e19 cm⁻³, andN_(A)<<[Mg]. The high Mg doping may be achieved by adjusting the growthconditions to promote the Mg incorporation into the solid phase, forexample by increasing the Mg/Ga ratio in the gas phase. The Mg andacceptor concentrations for embodiments of p-type transition layer 24are shown in region 32. Region 32 of FIG. 3 illustrates an approximaterange of Mg and acceptor concentrations. In some embodiments, the Mg andacceptor concentrations of p-type transition layer 24 may be outside ofregion 32. Transition layer 24 forms an Ohmic contact with metals, e.g.transparent or non-transparent contacts of Au, Ni, Al, Pt, Co, Ag, Ti,Pd, Rh, Ru, Re, and W, or alloys thereof.

[0022] The p-type dopants for p-type transition layer 24 are selectedfrom the Group II family which includes Be, Mg, Ca, Sr, Zn, and Cd. Apreferred dopant is Mg, which may be co-doped with a Group VIA element,such as O, S, Se, and Te.

[0023] In one example of the first embodiment, the thickness of p-typetransition layer 24 ranges between about 10 and about 200 nm. As aconsequence, the contribution of p-type transition layer 24 to theseries resistance is negligible. FIG. 4B demonstrates the I-Vcharacteristics for a Ni/Au metal p-type GaN contact in “back-to-back”(metal-semiconductor-semiconductor-metal) configuration for a Mg-dopedGaN layer optimized for Ohmic contact formation (p-type transition layeraccording to the first embodiment). The forward current (I) exhibits alinear dependence on the voltage (V) indicating that the contact isOhmic. FIG. 4A demonstrates the situation for a p-type conductivitylayer. The I-V curve indicates the presence of a barrier to currentflow.

[0024] The p-type transition layer forms a contact with the metal layerthat exhibits a barrier height<about 0.5 eV and almost Ohmiccharacteristics. If the contact is formed by depositing the metaldirectly on the p-type conductivity layer the barrier height is>about1.0 eV. Utilization of contacts with such high barrier height wouldincrease the forward voltage of the diodes and reduce their total powerefficiency. In FIG. 5A, the barrier height is illustrated as a functionof bulk resistivity of the transition material. Mg-doped GaN layers thathave a low resistivity exhibit a high barrier height when combined witha metal layer to form a contact. As illustrated in FIG. 5A, a preferredembodiment of the p-type transition layer, that is, an embodiment with abarrier height less than about 0.5 eV, exhibits a bulk resistivitybetween about 7 Ωcm and about 250 Ωcm. Such p-type transition layershave the smallest impact on the driving voltage of the device. Thedriving voltage of such devices is less than or equal to about 3.5volts. In other embodiments, the bulk resistivity may be greater than250 Ωcm. The differences in barrier heights of the p-type transition andconductivity layer may be explained by differences in the out-diffusionof Mg, redistribution of hydrogen near the surface of the Mg-doped GaNfilms, different properties of the surface, or formation of magnesiumnitride inclusions in the highly Mg-doped transition layer.

[0025] The barrier height may be further lowered through contactannealing. FIG. 5B demonstrates the effect of contact annealing in a RTAsystem on the barrier height of contacts formed with p-type conductivitylayers and contacts formed with p-type transition layers. The y-axisshows the barrier height, and the x-axis shows the temperature of athermal anneal to activate the p-type dopant in either type of layer.FIG. 5B thus illustrates the effect of two different anneals, a thermalacceptor-activation anneal and a contact anneal, called “contact RTA” onFIG. 5B.

[0026] Contact annealing reduces the barrier heights for both thetransition layer and the p-type conductivity layer. For example, thebarrier height for a p-type conductivity layer thermally annealed at600° C. drops from about 2.7 eV before the contact anneal to about 2.3eV after the contact anneal, and the barrier height for a p-typetransition layer thermally annealed at 600° C. drops from about 0.8 eVbefore the contact anneal to about 0.4 eV after the contact anneal.However, even after contact annealing, contacts formed with p-typeconductivity layers exhibit significantly higher barrier heights thancontacts formed with p-type transition layers, e.g. about 2.3 eV for ap-type conductivity layer compared to about 0.4 eV for a p-typetransition layer. Thus, though contact annealing does reduce the barrierheight for a p-type conductivity layer contact, the effect is not enoughto reduce the barrier height to that of a p-type transition layercontact.

[0027] The results shown in FIG. 5B also show that the observed barrierheight reductions are not strongly dependent on the temperature of theacceptor activation process, working equally well for activation at 600°C. and 850° C. The method to reduce the barrier height by contactannealing is described by Nakamura et al., Appl. Phys. Lett. 70, 1417(1997) “Room-temperature continuous-wave operation of InGaNmulti-quantum-well structure laser diodes with a lifetime of 27 hours”.

[0028] In a second embodiment, p-type transition layer 24 is not limitedto Mg doped GaN, but is any III-V material. P-type transition layer 24according to the second embodiment is homogeneously doped. P-typetransition layer 24 according to the second embodiment may be, forexample, InN, InGaN, AlInGaN, AlN, or AlGaN. When p-type transitionlayer 24 is InGaN, typically the group III compounds in the crystal areless than about 40% In, but the amount of In can range from 0-100% ofthe group III compound. When p-type transition layer 24 is AlGaN,typically the group III compounds in the crystal are less than about 20%Al, but the amount of Al can range from 0-100% of the group IIIcompound.

[0029]FIG. 6 illustrates the relationship between band gap and latticeparameter for compositions of aluminum, indium, gallium, and nitrogen.In FIG. 6, the squares represent the binary compounds AlN, GaN, and InN,the lines connecting the squares represent the ternary compounds AlGaN,AlInN, and InGaN with varying compositions of each group III material,and the shaded triangle between the lines represents the quaternarycompound AlInGaN with varying compositions of each group III material.Line 60 represents an example lattice constant. The dots represent thecomposition of potential LED device layers. The injection layer refersto p-type conductivity layer 22. The simplest devices to fabricate havereasonably close lattice constants for each of the device layers. Thus,FIG. 6 illustrates that once the compositions of device layers have beenselected, the composition of the p-type transition layer may be selectedto lattice-match the p-type transition layer to the device layers, andto optimize the p-type transition layer for Ohmic contact.

[0030] P-type transition layer 24 according to the second embodiment mayalso be any III-nitride arsenide compound, III-nitride phosphidecompound, or III-nitride arsenide phosphide compound, such as GaNAs,GaNP, or GaNAsP. The addition of even a small amount of As or P cansignificantly lower the bandgap of III-nitride semiconductors.

[0031] In the first and second embodiments of the invention, the p-typelayers of the device are homogeneously doped. In the third and fourthembodiments, described below, at least one of the p-type layers of thedevice has a varying concentration of dopant.

[0032]FIG. 7 illustrates a third embodiment of the invention wheretransition layer 24 (FIG. 2) is a doping superlattice. In many III-Vsemiconductors, a doping superlattice can achieve higher levels ofdoping than homogeneously doped layers. This is because in many III-Vsemiconductors, a heavily p-doped thick device layer exhibits poorsurface quality. Accordingly, heavily doped layers and lightly doped orundoped layers are alternated in order to form a heavily doped structurewith improved surface characteristics.

[0033] In a first example of the third embodiment, transition layer 24consists of sets 70 of alternating highly doped and lightly doped orundoped layers. Each set of layers 70 has a layer 71 of highly Mg-dopedmaterial on the bottom and a layer 72 of undoped or lightly Mg-dopedmaterial on the top. The designations “bottom” and “top” are arbitrary,such that either type of sublayer may be adjacent to both the p-typeconductive layer and the metal layer. Sublayers 71 and 72 range inthickness from 1 nm to 20 nm. In one example of the third embodiment,each of sublayers 71 and 72 is about 10 nm thick and transition layer 24includes 10 sets of sublayers such that transition layer 24 is 200 nmthick. Highly doped layer 71 has a Mg concentration ranging from about1e20 cm⁻³ to about 5e21 cm⁻³. Lightly doped layer 72 has a Mgconcentration ranging from undoped to about 1e20 cm⁻³.

[0034] In a second example of the third embodiment, rather than aheavily doped layer, layer 71 is a layer of elemental dopant. Thus, inthis example, layer 72 may be Mg-doped or undoped GaN or AlInGaN andlayer 71 may be elemental Mg.

[0035]FIG. 8 illustrates a fourth embodiment of the invention. A p-typelayer 28 separates active InGaN region 18 and metal layer 26B. P-typelayer 28 is between 5 nm and 200 nm thick. P-type layer 28 is doped toprovide for Ohmic contact formation with metal layer 26B and holeinjection into active region 18. The composition and concentration arevaried through layer 28. The variable doping in p-type layer 28eliminates the need for a separate p-type conductivity layer.

[0036]FIG. 9 illustrates one example of varying Mg content and fourexamples, labeled A-D, of varying composition in p-type layer 28,according to the fourth embodiment. Curve 82 illustrates one example ofthe Mg concentration in layer 28. The amount of Mg in layer 28 increasesfrom about 1e19 cm⁻³ in the region adjacent to active region 18 to about1e20 cm⁻³ in the region adjacent to metal layer 26B. The concentrationof Mg in the region of layer 28 adjacent to active region 18 may varyfrom about 1e18 cm⁻³ to about 5e19 cm⁻³. The concentration of Mg in theregion of layer 28 adjacent to metal layer 26B may vary from about 5e19cm⁻³ to about 1e21 cm⁻³. Curve 81 of example A illustrates a firstexample of varying composition where Al composition of layer 28 isvaried. The amount of Al in layer 28 decreases from about 20% in theregion adjacent to active region 18 to about 0% in the region adjacentto metal layer 26B. The presence of Al provides for efficient holeinjection into the active layer, thus the Al composition isadvantageously maximized in the region of layer 28 adjacent to activeregion 18. Curve 83 of example B illustrates an example where the Incomposition in layer 28 is varied. The amount of In increases from aboutzero percent in the region adjacent to active region 18 to about 40% inthe region adjacent to metal layer 26B. The presence of In lowers thebandgap of the material and thereby provides for efficient Ohmiccontact, thus the In composition is maximized near the metal contact.There may be no In present in the portion of the layer adjacent theactive layer.

[0037] In examples C and D, both the Al and the In compositions arevaried. In example C, as the composition of Al is reduced, the Al isreplaced with In. As illustrated in curve 84, the composition of Al iszero near metal contact 26B. Similarly, as illustrated in curve 85, thecomposition of In is zero near active region 18. Thus, layer 28 variesfrom AlGaN immediately adjacent to the active region, to AlInGaN in theregion between the active region and the metal contact, to InGaNimmediately adjacent to the metal contact. In example D, both Al and Inare present in all areas of layer 28. As illustrated by curve 86, the Alcomposition is reduced from the active region to the metal contact, butnever reaches zero composition. Similarly, as illustrated by curve 87,the In composition is reduced from the metal contact to the activeregion, but never reaches zero composition. Layer 28 is thus entirelyAlInGaN, but varies from more Al than In near the active region to moreIn than Al near the metal contact.

[0038]FIG. 9 illustrates just a few examples of the variation ofcomposition and concentration in p-type layer 28 according to the fourthembodiment. In other examples, Al is present only in the half of p-typelayer 28 adjacent to active region 18 and In is present only in the halfof p-type layer 28 adjacent to metal contact 26B. In other examples, thecomposition of other group III or group V elements are varied. In stillother examples, the concentration of a dopant other than Mg is varied.Further, As and P may be added to layer 28 to reduce the bandgap oflayer 28, typically in the region of layer 28 that is adjacent to thecontact. In order to form good contact, the lowest bandgap material isplaced next to the metal contact. Since As and P reduce the bandgap ofthe material, As and P are added to the 1 to 2 nm of layer 28 adjacentto the contact in order to improve the characteristics of the contact.In devices which incorporate As or P into p-type layer 28, As or P mayaccount for less than 3% of the of the group V materials.

[0039] While particular embodiments of the present invention have beenshown and described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention. For example,while the layer is illustrated as having been grown by MOCVD, it mayalso be fabricated by the techniques of MBE, HVPE, as well asevaporation, sputtering, diffusing, or wafer bonding.

What is being claimed is:
 1. A method of manufacturing a light-emittingdiode comprising: forming an n-type layer of GaN over a substrate;forming an active region over the n-type layer; forming a p-type layerover the active layer, the p-type layer having a varying composition anda varying concentration of a dopant; forming an n-type contact and ap-type contact, the n-type contact being connected to the n-type layer,the p-type contact being connected to the p-type layer.
 2. The method ofclaim 1, further comprising doping the p-type layer with a Group IIdopant selected from Be, Mg, Ca, Sr, Zn, Cd, and C.
 3. The method ofclaim 1, wherein the Group II dopant is magnesium, the method furthercomprising doping the p-type layer with a co-dopant selected from agroup consisting of Si, Ge, O, S, Se, and Te.
 4. The method of claim 1,further comprising: doping a region of the p-type layer adjacent to theactive region to a first concentration; and doping a region of thep-type layer adjacent to the p-type contact to a second concentration,wherein the first concentration is less than the second concentration.5. The method of claim 1 further comprising varying a composition ofaluminum from 20% in a region of the p-type layer adjacent to the activeregion to 0% in a region of the p-type layer adjacent to the p-typecontact.
 6. The method of claim 1 wherein the p-type layer is asuperlattice, and wherein forming a p-type layer further comprises:forming a first sublayer of doped p-type material; and forming a secondsublayer of doped p-type material, wherein a concentration of dopant inthe second sublayer is less than a concentration of dopant in the firstsublayer.
 7. A light-emitting diode comprising: a substrate; an n-typelayer of GaN, formed over the substrate; an active region, formed overthe n-type layer; a p-type layer, formed over the active layer, thep-type layer having a varying composition and a varying concentration ofa dopant; an n-type contact and a p-type contact, the n-type contactbeing connected to the n-type layer, the p-type contact being connectedto the p-type layer.
 8. The light-emitting diode of claim 7, wherein thep-type layer dopant is a Group II dopant selected from Be, Mg, Ca, Sr,Zn, Cd, and C.
 9. The light-emitting diode of claim 8, wherein the GroupII dopant is magnesium and the p-type layer further comprises aco-dopant selected from Si, Ge, O, S, Se, and Te.
 10. The light-emittingdiode of claim 7, wherein the p-type layer comprises a material selectedfrom III-nitride, III-nitride arsenide, III-nitride phosphide, andIII-nitride arsenide phosphide.
 11. The light-emitting diode of claim 7,wherein the p-type layer has a thickness between 5 and 200 nm.
 12. Thelight-emitting diode of claim 7, wherein a first concentration of thedopant in a region of the p-type layer adjacent to the active region isless than a second concentration of the dopant in a region of the p-typelayer adjacent to the p-type contact.
 13. The light-emitting diode ofclaim 12, wherein the dopant is magnesium and the first concentration isabout 1e18 cm⁻³ to about 5e19 cm⁻³.
 14. The light-emitting diode ofclaim 12, wherein the dopant is magnesium and the second concentrationis about 5e19 cm⁻³ to about 1e21 cm⁻³.
 15. The light-emitting diode ofclaim 7 wherein the p-type layer comprises a varying composition ofaluminum.
 16. The light-emitting diode of claim 15 wherein thecomposition of aluminum varies from about 20% in a region of the p-typelayer adjacent to the active region to about 0% in a region of thep-type layer adjacent to the p-type contact.
 17. The light emittingdiode of claim 7 wherein the p-type layer comprises a varyingcomposition of indium.
 18. The light emitting diode of claim 17 whereinthe composition of indium varies from about 0% in a region of the p-typelayer adjacent to the active region to about 40% in a region of thep-type layer adjacent to the p-type contact.
 19. The light emittingdiode of claim 7 wherein a driving voltage of the light emitting diodeis less than about 3.5 volts.